The present invention relates to a fixing apparatus that fixes unfixed images, and more particularly to one wherein a curl on a recording paper generated after fixing is corrected.
In a fixing apparatus, especially in an apparatus wherein heat is used for fixing, when a recording paper having toner images on the obverse thereof is given heat whose amount is larger than that given on the reverse of the paper only on its obverse or on the space on the side of the obverse, it is considered that the moisture in the recording paper is expelled unevenly between the obverse and the reverse of the recording paper, and due to a `bimetal-effect`, the paper curls heavily, while changing in its length and width depending on the amount of moisture expelled.
The curl mentioned above is one of the factors to cause paper jamming and deterioration of paper transportability in duplex copying in an electrophotographic apparatus.
In a fixing apparatus of a thermal fixing roller type, in particular, a recording paper curls more heavily toward the heat source side because the recording paper touches the heat source regardless of whether toner exists on the recording paper or not. Further, in the case of a fixing apparatus composed of a pair of a rigid roller and an elastic roller, a recording paper is given, at the location where the recording paper contacts the roller, a mechanical external force that forcibly bends the recording paper toward the rigid roller, thus, the curl is accelerated.
From the viewpoint mentioned above, there has been developed a fixing apparatus wherein the heating source is provided not on the side of a rigid roller but on the side of an elastic roller, and toner images contact the rigid roller and a recording paper is heated from its reverse side. Nevertheless, a fixing apparatus of this type still has a disadvantage that the thermal efficiency is low, though a recording paper is less curled.
Further, Japanese Patent Publication Open to Public Inspection No. 34572/1984 has disclosed a curl prevention means wherein a guide member having a curved surface that is curved in the direction opposite to the curl of a recording paper is provided just behind the heating source or the pressing source, thereby the curl generated on the recording paper is corrected, before the curl is set, to the original state.
In a fixing apparatus provided with aforesaid curl-prevention means, however, there occurs a phenomenon that coagulated toners and paper dust stick to the curved surface of aforesaid curl-prevention means when it is used for a long time, because images on the recording paper are kept in contact with the curved surface of the curl-prevention means.
Coagulated toners and paper dust sticking to the curved surface of a curl-prevention means cause soil on the image surface of the recording paper. In addition to that, when the curvature is increased, there is a fear that the curl-prevention means having aforesaid curved surface deviates with buckling from the paper-transport path in the fixing apparatus.
An object of the invention is to offer a fixing apparatus wherein the curl of a recording paper generated in the course of image-fixing is corrected, and thus the troubles caused by the aforesaid curl such as occurrence of paper jamming and deterioration of paper transportability in duplex copying in an electrophotographic apparatus, for example, are eliminated.